1. Field of the Invention
The present invention relates to a vehicle motion control device for controlling a motion of a vehicle by controlling braking force exerted on each wheel of the vehicle.
2. Description of the Prior Arts
There has conventionally been a demand for controlling a vehicle motion so as to prevent the turning state of the vehicle from being unstable due to the occurrence of an excessive roll angle on the vehicle, when the vehicle is in the turning state. This roll angle depends generally upon the magnitude of an actual lateral acceleration (accordingly, the centrifugal force exerted on the vehicle) that is a component of an acceleration actually exerted on the vehicle in the side-to-side direction of the vehicle body, whereby the centrifugal force exerted on the vehicle is decreased by forcibly generating a yawing moment in the direction opposite to the turning direction of the vehicle or by decelerating the vehicle.
Further, it is also conventionally required to control a vehicle motion so as to prevent that a turning state from being unstable since the turning state of the vehicle is brought into an excessive understeer (hereinafter referred to as “US”) state or excessive oversteer (hereinafter referred to as “OS”) state. This understeer state can be eliminated by forcibly generating a yawing moment on the vehicle in the direction same as the turning direction. On the other hand, the oversteer state can be eliminated by forcibly generating a yawing moment on the vehicle in the direction opposite to the turning direction.
From the above, a vehicle motion control device disclosed in the patent document 1 executes a roll-over preventing control for exerting roll-over preventing braking force, that is for generating a yawing moment on the vehicle in the direction opposite to the turning direction, on the front wheel at the outer side of the turning direction, when the vehicle is in the turning state and a degree of a tendency that an excessive roll angle occurs indicated by a predetermined excessive roll angle occurrence tendency index value (for example, an absolute value of the actual lateral acceleration or the like) exceeds a predetermined degree (for example, when the absolute value of the actual lateral acceleration exceeds a predetermined actual lateral acceleration threshold value).
[Patent Document 1]
Japanese Unexamined Patent Application No. HEI10-119743
Further, this device executes an understeer restraining control for exerting understeer restraining braking force, that is for generating a yawing moment on a vehicle in a direction same as the turning direction, on a rear wheel at the inner side of the turning direction, when a degree of understeer tendency indicated by a predetermined OS-US tendency index value (for example, a deviation between a target lateral acceleration and an actual lateral acceleration (lateral acceleration deviation)) exceeds a predetermined degree (for example, when the lateral acceleration deviation exceeds a positive lateral acceleration deviation threshold value), and executes an oversteer restraining control for exerting oversteer restraining braking force, that is for generating a yawing moment on a vehicle in a direction opposite to the turning direction, on a front wheel at the outer side of the turning direction, when a degree of oversteer tendency indicated by the predetermined OS-US tendency index value exceeds a predetermined degree (for example, when the lateral acceleration deviation falls below a negative lateral acceleration deviation threshold value). This device can simultaneously execute the above-mentioned each control.
Here, the understeer restraining control and the oversteer restraining control are executed based upon the same index value (the above-mentioned OS-US tendency index value), and there is a relationship that the OS-US tendency index value upon executing the understeer restraining control and the OS-US tendency index value upon executing the oversteer restraining control are not overlapped with each other. Therefore, the understeer restraining control and the oversteer restraining control are not simultaneously executed. On the other hand, the roll-over preventing control is executed based upon the excessive roll angle occurrence tendency index value that is different from the OS-US tendency index value, so that the roll-over preventing control and the understeer restraining control can be simultaneously executed.
Meanwhile, the roll-over preventing control is a control for generating a yawing moment on the vehicle in the direction opposite to the turning direction, while the understeer restraining control is a control for generating a yawing moment on the vehicle in the direction same as the turning direction, as described above. Specifically, the roll-over preventing control and the understeer restraining control have a directional property of the control that is reverse to each other. Therefore, in the disclosed device wherein the roll-over preventing control and the understeer restraining control can be simultaneously executed, either one of the controls increases control volume of the other one since both controls are simultaneously executed, so that both controls are dissipated. As a result, there arises a problem that the turning state of the vehicle may be unstable.